In general, it is desirable that material used as an electromagnetic wave absorber exhibit a complex dielectric constant in the operating frequency ranges. More specifically, the constant should have a relatively small real dielectric constant and a large dielectric loss factor in operating ranges. Accordingly, composite materials which contain a mixture of conductive material and non-conductive material are used in the manufacture of electromagnetic wave absorbers. The real dielectric constant of complex dielectric constant of the composite material increases in proportion to the weight of conductive material added. In contrast, the imaginary dielectric loss factor increases only slightly until the amount of conductive material in the composite is increased to a threshold concentration which is sufficient to permit contact among the individual particles of the conductive material. In short, the presence of conductive material in concentrations over the threshold amount generally results in an abrupt increase of the dielectric loss factor of the dielectric constant with a corresponding reduction in electric resistance.
In order to produce a composite material having the desired complex dielectric constant, only a small weight of a conductive material is used. This permits control of the specific conductance of the resultant composite material at a predetermined reduced value.
Electromagnetic wave absorbers which are now in wide use generally use carbon black as a conductive material and a synthetic resin matrix as a non-conductive material. However, carbon black particles are not uniform and tend to form agglomerates, thus making it difficult to form uniform mixtures of carbon black in the non-conductive material. Accordingly, these composite materials had local irregulartities in electric characteristics. Thus, it has been substantially difficult to increase the dielectric loss factor of the complex dielectric constant without inviting an increase of the real dielectric constant.
Under these circumstances, there has been a strong demand for an electromagnetic wave absorbing composite material which can exhibit the desired complex dielectric constant.